1. Technical Field
The present invention relates to an apparatus and a method for separating and concentrating a specified gas from a gas to be treated containing a plurality of gases.
2. Related Art
Conventionally, in a semiconductor manufacturing process, a liquid crystal display manufacturing process, or the like, various gases are used according to the process. For instance, in a dry-etching step, a thin-film-forming step, or the like, gases of PFC (perfluoro compound) that is a compound containing fluorine such as CF4, NF3, C2F6, C3F8, SF6, CHF3, and COF2, are used as a reactive gas, and an exhaust gas including them is produced.
An exhaust gas including the PFC gases is treated with various methods, because the gas has a high global warming potential and it is not preferable to discharge the exhaust gas outside in an untreated state. Such a treatment method includes a process of decomposing the PFC gas and removing its harmful effect by abatement treatment such as of a combustion type, a catalytic type, an adsorption type or a plasma decomposition type. Further, as a low-cost recovery method, there has been proposed a chromatographic separation method in which PFC gases are separated from an exhaust gas by employing a chromatographic column containing various packing materials.
For instance, Japanese Patent Laid-Open Publication No. 2002-273144 proposes a gas separation apparatus which is directed to separating a specified gas from a gas to be treated containing the specified gas comprising a plurality of components such as PFCs, and has a separation unit for chromatographically separating the specified gas from the gas to be treated by using a column packed with activated carbon.
In general, when a PFC gas is exhausted, the PFC gas is diluted with a large amount of nitrogen gas or the like so as to protect an exhaust line, a vacuum pump, or the like, and is subsequently exhausted. Accordingly, the PFC gas is diluted to a concentration as low as several percent, though this depends on conditions. For this reason, when performing the above-described harmful substance removal treatment, a PFC concentration process carried out by removing nitrogen or the like from the exhaust gas is necessary in order to eliminate nitrogen or the like that is unnecessary for the harmful substance removal treatment and to thereby increase treatment efficiency. Further, when recovering a PFC, a PFC concentration process is necessary in order to eliminate impurities such as nitrogen from the recovered PFC.
As conventional methods for separating and concentrating a specified gas such as a PFC from a gas to be treated containing a plurality of gas components, there have been proposed a membrane treatment method of separating the specified gas from the gas to be treated through a membrane, and a cryogenic distillation method of separating the specified gas by utilizing the differences in boiling points between the gases within the gas to be treated.
A membrane treatment method can separate a PFC gas from nitrogen to some extent, but the concentration of the separated PFC is only about 90% even after having been separated by multistage membrane treatment. Because this concentration is lower than the concentration of a fresh PFC gas (normally approximately 99.99%), it is difficult to reuse the separated PFC gas without performing additional treatment. Moreover, a membrane treatment method is disadvantageous in that a plurality of PFC gas components cannot be separated individually by each component. Furthermore, a cryogenic distillation method has many drawbacks, including the disadvantage that this method requires a large-scale facility and extremely high running costs, and the disadvantage that the method cannot be applied to a CF4/NF3 system or the like in which the boiling point difference is only approximately 1° C.